Chapter 4 Signal ConnectionsPCI E Series User Manual4-36 ©National Instruments CorporationThe sample interval counter on the PCI E Series board normallygenerates the CONVERT* signal unless you select some externalsource. The counter is started by the STARTSCAN signal and continuesto count down and reload itself until the scan is finished. It then reloadsitself in preparation for the next STARTSCAN pulse.A/D conversions generated by either an internal or externalCONVERT* signal are inhibited unless they occur within a DAQsequence. Scans occurring within a DAQ sequence may be gated byeither the hardware (AIGATE) signal or software command registergate.AIGATE SignalAny PFI pin can externally input the AIGATE signal, which is notavailable as an output on the I/O connector. The AIGATE signal canmask off scans in a DAQ sequence. You can configure the PFI pin youselect as the source for the AIGATE signal in either the level-detectionor edge-detection mode. You can configure the polarity selection for thePFI pin for either active high or active low.In the level-detection mode if AIGATE is active, the STARTSCANsignal is masked off and no scans can occur. In the edge-detectionmode, the first active edge disables the STARTSCAN signal, and thesecond active edge enables STARTSCAN.The AIGATE signal can neither stop a scan in progress nor continue apreviously gated-off scan; in other words, once a scan has started,AIGATE does not gate off conversions until the beginning of the nextscan and, conversely, if conversions are being gated off, AIGATE doesnot gate them back on until the beginning of the next scan.SISOURCE SignalAny PFI pin can externally input the SISOURCE signal, which is notavailable as an output on the I/O connector. The onboard scan intervalcounter uses the SISOURCE signal as a clock to time the generation ofthe STARTSCAN signal. You must configure the PFI pin you select asthe source for the SISOURCE signal in the level-detection mode. Youcan configure the polarity selection for the PFI pin for either active highor active low.
